X-Men #4: "they hate us -- fear us"

The fourth issue of X-Men bursts with ideas new and old.  It features the first return of a villain - Magneto, who has assembled a "brotherhood of evil mutants" around him (a descriptive term which does not appear to be used by Magneto himself), going up another floor on the threat escalator. Although his identity is given away in the cover, the interior pages introduce his minions first, who refer merely to their "leader", and so avoids mentioning Magneto by name until his appearance on panel in page 6. We meet uncouth Toad, impulsive Pietro, proud, compassionate, Wanda and the aggressive Mastermind.

More importantly for the saga, Magneto is given a philosophical justification of sorts. He claims his coup d'état in Santo Marco is necessary as homo sapiens hate and fear mutants, and they would "would kill [mutants] if only they could! We only fight in self defense!" Coupled with this, we see the first example of actual anti-mutant prejudice, in the flashback to Pietro and Wanda's origin story. An odd result of this is that Wanda's codename, "Scarlet Witch" was given to her by a villager from the lynch mob in the "heart of Europe". Her use of it is a reclamation of the term, and at this stage her powers are no more inherently magical than any those of any other mutant.

Then the X-Men arrive and ruin everything, defeating the Brotherhood after a series of thrilling fights. Pietro, moved by Wanda, disables the scorched earth bomb that Magneto has left behind, saving the innocent civilian population of Santo Marco. And, as a last gasp, Professor X incurs an enormous strain, resulting in his depowerment!

Continuity notes

First appearances of Toad, Quicksilver (Pietro), Scarlet Witch (Wanda), Mastermind, along with the return of Magneto. Pietro and Wanda are given an origin story, implied to have happened some time between issue #1 and #4. Xavier is depowered!